PROJECT ABSTRACT Visceral hypersensitivity is frequently observed in a subpopulation of irritable bowel syndrome (IBS) patients. The responsible mechanism is unclear. Clinical studies show that many of these patients display subclinical signs of mucosal inflammation accompanied by increased gut permeability. It is therefore conceivable that impaired mucosal barrier function may facilitate increased antigen presentation to the immune cells in the submucosa, resulting in inflammation. Mast cells been shown to play important roles in the innate immune defense by producing proinflammatory agents and pain mediators which may induce visceral hypersensitivity. Our preliminary data show that resolvins, a novel class of endogenous anti-inflammatory lipid mediators derived from omega-3 polyunsaturated fatty acids, are present in the colonic mucosa and play an important role in regulating submucosal mast cell function and modulating the sensory threshold. Furthermore, our clinical studies show that in IBS-D patients with gut dysbiosis, the level of colonic resolvin D1 (RvD1) is significantly reduced compared to that observed in healthy controls. We also observed that resolvin-deficient mice exhibit evidence of visceral hypersensitivity. The objectives of our studies are 1. to investigate the functional relationship between the level of resolvins in the colonic tissue and visceral mechanical sensitivity; and 2. to examine the mechanisms by which resolvins inhibit mucosal inflammation and reduce visceral hypersensitivity. We hypothesize that the level of RvD1 in the colonic tissue modulates visceral mechanical sensitivity. We further propose that RvD1 mechanistically controls mast cell activation and synthesis of proinflammatory mediators, and decreases the excitability of sensory neurons, preventing the development of visceral hypersensitivity. These beneficial actions of RvD1 are mediated through the action of formyl peptide receptor 2 (FPR2)/ G?i to reduce cAMP formation. To test this hypothesis, we have 3 specific aims: Aim 1: Using 2 rodent models in which colonic RvD1 levels are modulated by dietary factors and gut microbiota, respectively, we aim to demonstrate that the levels of RvD1 in the colonic tissue play an important role in regulating visceral mechanical sensitivity; Aim 2: To show in vivo and in vitro that RvD1 inhibits the degranulation of mast cells and prevents submucosal inflammation and the development of visceral hypersensitivity. Aim 3: To demonstrate that RVD1 modulates the excitability of gut-protecting DRG neurons by activating the FPR2 receptor, which in turn reduces intracellular cAMP levels. Results from these studies will support the use of resolvins as a class of novel therapeutic agents to reduce submucosal inflammation and decrease pain in IBS patients.